Question

Which of the following factors may be regarded as the main cause of lanthanide contraction?

• A. Poor shielding of one of 4f electron by another in the sub-shell.
• B. Effective shielding of one of 4f electrons by another in the subshell
• C. Poorer shielding of 5d electrons by 4f electrons
• D. Greater shielding of 5d electrons by 4f electrons

Hint:

The 4f electrons shield one another poorly.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Lanthanide contraction is the steady decrease in atomic and ionic radii from La to Lu. As we move across, one proton is added to the nucleus and one electron goes into the \(4f\) sub-shell each time.

Step 2: Why the size falls:
The \(4f\) orbitals have diffuse shapes, so one \(4f\) electron shields another poorly from the nuclear charge. As a result the effective nuclear charge rises steadily and pulls the outer electrons in, which shrinks the size.

Step 3: Check the options:
(A) says poor shielding among \(4f\) electrons, which is the cause. (B) says effective shielding, which would stop the contraction. (C) and (D) talk about \(5d\) electrons being shielded by \(4f\), which is not the main cause. So (A) is correct.

Final Answer:
Poor shielding by 4f electrons causes lanthanide contraction. \[ \boxed{A} \]